Limiting magnitude of reference source for shift-and-add

The shift-and-add method described by e.g. Christou (1991) bears the greatest similarity to the Lucky Exposures method, the principle difference being that all the short exposures are used rather than just those exposures with the highest Strehl ratio. In order for the re-centring to be successful using exposures which have a lower Strehl ratio, a correspondingly brighter reference source is required in order provide the same number of photons within the brightest speckle. The limiting magnitude of reference source for this technique is thus one or two magnitudes poorer than that for Lucky Exposures.